Capture std::clog writes and combine them with std::cerr writes (#989)

This also introduces Catch::clog() method to allow embedded targets
to override std::clog usage with their own stream (presumably null-sink),
similarly to how Catch::cout() and Catch::cerr() are used.

Fixes #989

projects/SelfTest/Baselines/junit.sw.approved.txt

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <testsuitesspanner>
-  <testsuite name="<exe-name>" errors="13" failures="78" tests="971" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
+  <testsuite name="<exe-name>" errors="13" failures="81" tests="974" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
     <testcase classname="global" name="# A test name that starts with a #" time="{duration}"/>
     <testcase classname="#748 - captures with unexpected exceptions" name="outside assertions" time="{duration}">
       <error type="TEST_CASE">
@@ -475,6 +475,13 @@ A string sent directly to stderr
       </system-err>
     </testcase>
     <testcase classname="global" name="Some simple comparisons between doubles" time="{duration}"/>
+    <testcase classname="Standard error is reported and redirected" name="Interleaved writes to cerr and clog" time="{duration}">
+      <system-err>
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
+      </system-err>
+    </testcase>
     <testcase classname="Standard output from all sections is reported" name="two" time="{duration}">
       <system-out>
 Message from section one
@@ -750,6 +757,9 @@ hello
     </system-out>
     <system-err>
 A string sent directly to stderr
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
     </system-err>
   </testsuite>
 </testsuites>